I don't have the manual for the PM4L but I have one for the PM2M which is similar in everything except that the display for the 2M is analog and when in white channel it has scale for both time and density, The 4L display only time value in the white channel. Color channels are display in densities.
Originally Posted by Mike Wilde
The response of the PMT (Photomultiplier Tube) is linear its output is routed thru a bunch of diode and turn to logarithmic response. I don't exactly know how they did it. This signal is then display on the meter. On the 4L the output once again converted back to linear when in the white channel via a Log to Linear IC.